1. Field of the Invention
The present invention is directed to a method and apparatus for analog-to-digital conversion of a chronologically changing analog electrical input signal.
2. Related Application
The subject matter of this application is related to the subject matter of an application filed simultaneously with the present application and having Ser. No. 07/662,403, and assigned to the same assignee as the present application.
3. Description of the Prior Art
Analog-to-digital converters in the form of integrated circuits are currently available which operate, for example, according to the "dual slope" method, and permit analog-to-digital conversion of a chronologically changing analog electrical input signal with a resolution of up to 16 bits. The use of analog-to-digital converters for some purposes, such as electrocardiography (ECG) or electroencephalography (EEG), require that such converters be operated so that the smallest acquirable voltage difference of the signal to be measured (which corresponds to the least significant bit (LSB) of the analog-to-digital conversion) is on the order of magnitude of a few microvolts. Under such conditions, known analog-to-digital converters have an extremely limited dynamic range for the signal to be measured. This results in the analog-to-digital converter being very quickly overdriven by the signal to be measured. This is particularly undesirable in the case of electrocardiography, wherein registration of an electrocardiogram should be possible immediately after a defibrillation. To effect defibrillation, the heart of a patient is charged with pulses having relatively high voltage and current intensity. It is conceivable to alleviate the problem of overdriving the analog-to-digital converter by using analog-to-digital converters having higher resolution (number of bits). If the analog-to-digital converter is to be executed as an integrated circuit, the manufacture of such an integrated converter having a sufficiently high resolution is extremely expensive, and the alternative of using a discretely constructed analog-to-digital converter involves a considerable technological and financial outlay.
Such outlay can be at least partially avoided by using so-called DAC feedback. Using this known technique, the digital output signals of the analog-to-digital converter are supplied to an electronic calculating means, which drives a digital-to-analog converter connected to the electronic calculating means so that it generates an analog output signal, which is then subtracted in a differential amplifier from the signal to be measured. The output from the differential amplifier is supplied to the analog-to-digital converter. The digital-to-analog converter is driven by the electronic calculating means dependent on the momentary amplitude of the signal to be measured, so that the output of the differential amplifier does not exceed the dynamic range of the analog-to-digital converter. The digital data corresponding to the output signal of the differential amplifier are subsequently corrected in the electronic calculating means by adding the corresponding value of the output signal of the digital-to-analog converter thereto. Exact results are only obtained with this method, however, if the resolution (number of bits) of the digital-to-analog converter corresponds to the desired resolution of the analog-to-digital conversion. Problems similar to those described above in conjunction with analog-to-digital converters also occur in digital-to-analog converters, therefore digital-to-analog converters are usually employed having a resolution which is lower than the desired resolution of the analog-to-digital conversion. This results in imprecisions in the conversion. For example, the results of the analog-to-digital conversion contain discontinuities which are not present in the analog input signal. A further disadvantage of this known technique is the necessity of an additional digital-to-analog converter, which results in additional costs.